1. Field of the Invention
The present invention relates to a photoelectric converting device having a photocharge accumulating region whose potential is controlled through a capacitor.
2. Related Background Art
As a conventional photoelectric converting system, for example, there is the system disclosed in the Official Gazette of EPC Application Laid-Open No. 0132076.
FIG. 1A is a schematic plan view of the conventional photoelectric converting device disclosed in this Official Gazette and FIG. 1B is a cross sectional view taken along the line I--I in FIG. 1A.
In these diagrams, photosensor cells are formed and arranged on a substrate 101 in a line. Each of the photosensor cells is electrically insulated from the adjacent photosensor cells by an element isolating region 102.
Each photosensor cell has the following constitution.
An n.sup.- epitaxial region (hereinafter, referred to as an n.sup.- region) 103 is formed on the substrate 101. A p region 104 and an n.sup.+ region 105 are formed on and over the n.sup.- region 103. The p region 104 and n.sup.+ region 105 serve as a base and an emitter of a bipolar transistor, respectively.
An oxide film 106 is formed over the n.sup.- region 103 formed with those respective regions in this manner. A capacitor electrode 107 having a predetermined area is formed on the oxide film 106. The capacitor electrode 107 faces the p region 104 through the oxide film 106. A potential of the p region 104 in the floating state is controlled by applying a pulse potential to the capacitor electrode 107.
In addition, an emitter electrode 108 connected to the n.sup.+ region 105 is formed. An electrode (not shown) to apply a potential to a collector of the bipolar transistor through an n.sup.+ region having a high impurity concentration is formed on the back surface of the substrate 101.
The fundamental operation will now be described. A light enters the p region 104 as the base of the bipolar transistor. The charges corresponding to the incident light amount are accumulated into the p region 104 (accumulating operation). The base potential is changed due to the charges accumulated. A current flowing between the emitter and collector is controlled due to the potential change, so that an electrical signal corresponding to the incident light amount can be obtained (reading operation). On the other hand, to remove the charges accumulated in the p region 104, the emitter electrode 108 is grounded and a pulse of a positive voltage is applied to the capacitor electrode 107 (refreshing operation). By applying the positive voltage, the p region 104 is forwardly biased with respect to the n.sup.+ region 105 and the charges accumulated are removed. Thereafter, the respective accumulating, reading, and refreshing operations are repeated.
In other words, according to the system proposed in conventional device, the charges generated due to the incident light are accumulated into the p region 104 as the base and the current flowing between the emitter electrode 108 and the collector electrode is controlled by the amount of accumulated charges. Therefore, after the accumulated charges were amplified due to the amplifying function of each cell, they are read out. Thus, the high power, high sensitivity, and further low noise can be accomplished.
A potential Vp generated in the base by the holes accumulated in the base due to the light excitation is given by Q/C (namely, Vp=Q/C), wherein Q denotes an amount of charges of the holes accumulated in the base and C is a capacitance connected to the base. As will be apparent from this expression, in the case where elements are highly integrated, both Q and C decrease together with the reduction in the cell size, so that the potential Vp which is generated due to the light excitation is held almost constant. Therefore, the system proposed in the conventional device is also advantageous for the future high resolution.
However, according to the conventional photoelectric converting device, in the case of applying the design rule of 2 .mu.m, for example, the contact portion on the emitter region 105 is limited to 2 .mu.m, so that the width of 10 .mu.m or more is needed per photo sensor cell. Therefore, in the case of arranging, e.g., 1000 cells in a line, the length of photoelectric converting device itself reaches 10 mm. Such a large device has the problems such that the yield deteriorates and the variation in characteristics of the cells becomes large. In addition, the resolution is limited by the design rule and it is difficult to attain high resolution.